Oxygen saturation, along with heart rate, breathing rate, blood pressure, and body temperature, is a vital physiological parameter. It is a relative measure of the oxygen amount dissolved or carried in a given medium, such as blood. It indicates whether a person has sufficient supply of oxygen and reflects the health level of the cardiorespiratory system. Continuous monitoring of oxygen saturation level is important in detecting hypoxemia under many medical situations, including anesthesia, sleep apnea, and parturition. It is employed in intensive care, operating room, emergency care, neonatal care, sleep study, and veterinary care [1].
Mixed venous oxygen saturation (SvO2), tissue oxygen saturation (StO2), and arterial oxygen saturation (SaO2) are a few major methods used for the determination of oxygen saturation levels in human body. SvO2 is a measurement of the oxygen remaining in the blood after passing through the capillary bed, which indicates moment-to-moment variation between oxygen supply and demand [2]. It can be monitored using fiber optics catheters. StO2 provides an assessment of tissue perfusion and it can be measured by near infrared (NIR) spectroscopy. SaO2 is a measurement of oxygen saturation in the arteries. An estimation of SaO2 at peripheral capillary is called SpO2, which is the primary focus of this paper. Unlike traditional SaO2 measurement, which is normally conducted invasively via a blood test with a blood gas analyzer, SpO2 can be measured by noninvasive methods. Monitoring SpO2 provides a quick and convenient assessment of user's oxygenation status. The most widely used device for SpO2 monitoring is pulse oximeter, which is often attached to the finger for measurement purpose. The hardware implementation of pulse oximetry includes two main components: 1) an illumination source usually composed of a dual-wavelength LED, and 2) a photodetector—typically a photodiode. SpO2 values typically range from 95% to 100% in healthy individuals. Continuous low SpO2 levels (<90%) may indicate an oxygen delivery problem [3].
Recent technological advances have enabled measurements of some of the physiological signals in noncontact ways [4]-[6]. Remote SpO2 detection provides people with a method to measure oxygen saturation noninvasively under normal daily setting. Absence of physical contact between the subject and the device allows for a more comfortable and less stressful measurement condition. The inaccurate SpO2 readings caused by varied pressure applied from finger to the contact sensor can also be avoided [7], besides preventing skin irritation that can occur in some individuals, especially infants, during extended monitoring periods. Noncontact pulse oximetry also provides a suitable SpO2 measurement alternative for individuals with finger injuries, or for those with poor peripheral perfusion or dark pigmentation on fingers, for whom traditional pulse oximetry may otherwise lead to inaccurate measurements [8].
In recent years, researchers have attempted different SpO2 measurement approaches using noncontact methods. For example, Humphreys et al. [9], [10] used a CMOS camera with LED arrays that emit two different wavelengths as the light source for noncontact pulse oximetry. Due to low frame rate and sensitivity to ambient light, the noise in the measured PPG signals was too large to obtain accurate SpO2 values. Wieringa et al. [11] also used a CMOS camera, but with three different wavelengths to investigate the feasibility of an “SpO2 camera.” However, no SpO2 results were presented due to poor SNR of the PPG signals. Kong et al. [12] used two CCD cameras, each mounted with a narrow bandpass filter to capture PPG signals at two different wavelengths (520 and 660 nm) in ambient lighting condition. The test only covered a narrow SpO2 range (97%-99%). For practical applications, such as clinical settings, it is necessary to be able to measure SpO2 over a wider range (at least 80%-100%). Tarassenko et al. [13] and Bal et al. [14] used a camera to calculate SpO2 based on the PPG information obtained from the RGB channels under ambient lighting condition. Other researchers have found that the PPG signals extracted from the red and blue channels were noisier than those extracted from the green channel [6], [15], [16]. Moreover, for digital cameras, each color channel (red, green, or blue) covers a band of optical spectrum [17] with a width of ˜100 nm, which is different from the traditional pulse oximetry method that uses monochromatic light sources with wavelengths selected to maximize the detection sensitivity of oxygenated and deoxygenated hemoglobin in blood. Tsai et al. [18], [19] used a CCD camera with red and infrared LEDs to take still images of hand and analyzed SpO2 by looking into the reflective intensity of the shallow skin tissue. These authors compared the SpO2 results against partial pressure of oxygen values (PaO2), instead of the standard pulse oximetry. Although they showed correlation between the results obtained using the two methods, a direct demonstration of SpO2 measurement is still lacking.
The present invention, in contrast to the above discussed methods, is a new noncontact method which is based on video recording of a subject's facial area to measure SpO2. To the best of the inventors' knowledge, this is the first demonstration of a low-cost video-based method with high temporal resolution and signal-to-noise ratio to accurately monitor wide range of SpO2 without any physical contact between the subject and the device. The contributions of this study include: 1) development of a hardware system with video capture and illumination synchronization control, 2) identification of optimized light sources to achieve accurate PPG and SpO2 detection when using noncontact method, 3) validation of method over a wide clinically relevant range of SpO2 via a pilot study of subjects, and 4) addition of SpO2 tracking to our previously reported noncontact physiological monitoring platform, which can detect heart rate, breathing pattern, and pulse transit time [4].